BMI Calculator
Body Mass Index compares your weight to your height using a single ratio. It's a fast screening number, not a diagnosis — it doesn't know your muscle mass, frame, or age. Use it as a starting point, not a verdict.
Enter your height and weight to see your result and healthy weight range.
How it's calculated
BMI = weight in kilograms ÷ (height in metres)². The formula dates to the 1830s, when Belgian mathematician Adolphe Quetelet built it for population-level statistics, not individual diagnosis — it treats a bodybuilder and a sedentary adult of the same height and weight identically. The World Health Organization's adult bands are: below 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 and above obese. Pregnant people, children, and very muscular or very tall/short adults should weigh this number lightly and talk to a clinician for anything more specific.
A worked example
Take someone who is 170 cm tall and weighs 68 kg. Convert height to metres (1.70 m), square it (2.89), then divide weight by that number: 68 ÷ 2.89 = 23.5. That falls inside the 18.5–24.9 healthy-weight band. Now compare a second person, also 170 cm but weighing 82 kg with a low body-fat percentage from years of strength training: 82 ÷ 2.89 = 28.4, technically "overweight" by BMI alone, despite likely being leaner and fitter than the first example. This is exactly the blind spot BMI has — it can't see the difference between muscle and fat, only total mass relative to height.
Where BMI holds up, and where it doesn't
BMI is genuinely useful as a fast, free, population-level screening tool — it correlates reasonably well with body fat percentage across large groups, and it's cheap to calculate compared to alternatives like DEXA scans or skinfold callipers. Public health bodies use it precisely because it scales to millions of people with just a tape measure and a scale.
Where it breaks down is at the individual level, especially for four groups: athletes and heavily muscled people (BMI overestimates their risk), older adults who've lost muscle mass with age (BMI can underestimate risk, since fat has quietly replaced muscle at the same weight), people of Asian descent (research suggests health risk rises at somewhat lower BMI thresholds than the standard WHO bands), and anyone carrying weight predominantly around the abdomen versus the hips — BMI can't see where fat sits, only how much of it there roughly is.
BMI alongside other measures
Because of these limits, most clinicians now treat BMI as a first-pass filter rather than a verdict — pairing it with waist circumference (a rough proxy for visceral, abdominal fat, which tracks more closely with cardiovascular risk), waist-to-height ratio, or body composition testing when something more precise is needed. If your BMI sits outside the "healthy" band, that's a prompt to look closer, not a diagnosis to accept at face value.
Frequently asked questions
What is a healthy BMI range?
For most adults, the World Health Organization defines a healthy BMI as 18.5 to 24.9. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These bands were built from population data and are a starting point for conversation, not a personal verdict.
Is BMI accurate for everyone?
No. BMI doesn't distinguish muscle from fat, so athletes and very muscular people can score higher than their actual body fat would suggest. It also doesn't account for age, sex, ethnicity, or where fat is distributed on the body — two people with the same BMI can have very different health profiles. It's most reliable as a population-level screening tool, less so as an individual diagnosis.
How is BMI calculated?
BMI is calculated as weight in kilograms divided by height in metres squared (kg/m²). For a 170 cm, 68 kg adult, that's 68 ÷ (1.70 × 1.70) = 23.5. This calculator does the metric-imperial conversion for you if you enter feet, inches, and pounds instead.
What should I do if my BMI is underweight?
A single underweight reading isn't automatically a problem — some people are naturally lean. But if it's a recent or unexplained drop, it's worth discussing with a doctor. For a practical, muscle-first approach to gaining weight, read our guide on healthy weight gain.
Why does BMI treat muscle and fat the same?
Because the formula only ever sees two numbers — total weight and height. It has no way to measure what that weight is made of. A kilogram of muscle and a kilogram of fat weigh exactly the same, even though they look and behave completely differently on the body, which is why two people can share a BMI and look nothing alike.
Should I use BMI or waist-to-height ratio?
They answer slightly different questions. BMI estimates overall mass relative to height; waist-to-height ratio focuses on abdominal fat specifically, which research links more directly to cardiovascular and metabolic risk. Using both together — BMI as a quick baseline, waist measurement as a sanity check — gives a fuller picture than either alone.